Root sorption

Ecological studies involving rare earth elements although not as extensive as biochemical studies, shows some promise in studies involving (i) sedimentation and erosion, (ii) ant behaviour, (iii) sorption by aquatic insects and (iv) root sorption. 

Organic chemicals can directly reach above ground plant tissues by vapor and particle deposition as discussed in Chapter 7, or they many reach below ground tissues by way of the roots, the emphasis of this chapter. Atmospheric deposition pathways to above ground plant components are likely to be more important for hydrophobic and less volatile compounds, while the root sorption and uptake pathway... 

A rationalization of the complex behavior of pectins in solutions and gels with respect to their stmcture, solvation, and the presence of ions and other saccharides has been presented (123). The solution and sorption properties of gum tragacanth and the pectin isolated from the roots of Hibiscus mani/)ot F (Tororoaoi) contributes to their use in specialty paper manufacture (124—126). 

Eor pesticides to leach to groundwater, it may be necessary for preferential flow through macropores to dominate the sorption processes that control pesticide leaching to groundwater. Several studies have demonstrated that large continuous macropores exist in soil and provide pathways for rapid movement of water solutes. Increased permeabiUty, percolation, and solute transport can result from increased porosity, especially in no-tiUage systems where pore stmcture is stiU intact at the soil surface (70). Plant roots are important in creation and stabilization of soil macropores (71). 

Preferential flow through root-mediated soil pores has been demonstrated for chloride, nitrate, and other ions that are not sorbed onto soil organic matter and clays. However, pesticide sorption onto soil affects both mobiUty of the pesticide as well as its residual life in the soil. Pesticide sorption onto root organic matter or organic linings of worm burrows may also slow transport of pesticides relative to water (72), thus countering the effects of increased permeabihty caused by roots. 

Maiti and Bhowmick also investigated the diffusion and sorption of methyl ethyl ketone (MEK) and tetrahydrofuran (THE) through fluoroelastomer-clay nanocomposites in the range of 30°C-60°C by swelling experiments [98]. A representative sorption-plot (i.e., mass uptake versus square root of time, at 45°C for all the nanocomposite systems is given in Figure 2.12. 

Root products may be classified into types on the basis of their (1) chemical properties, such as composition, solubility, stability (e.g., hydrolysis, oxidation), volatility, molecular weight etc. (2) site of origin and (3) e.stablished, not just perceived, functions. The chemical properties determine in turn their biological activity and how the compounds will behave in soils their persistence in soil is very much an outcome of their chemical behavior, particularly sorption and their biodegradability. 

In soil, the chances that any enzyme will retain its activity are very slim indeed, because inactivation can occur by denaturation, microbial degradation, and sorption (61,62), although it is possible that sorption may protect an enzyme from microbial degradation or chemical hydrolysis and retain its activity. The nature of most enzymes, particularly size and charge characteristics, is such that they would have very low mobility in soils, so that if a secreted enzyme is to have any effect, it must operate close to the point of secretion and its substrate must be able to diffuse to the enzyme. Secretory acid phosphatase was found to be produced in response to P-deficiency stress by epidermal cells of the main tap roots of white lupin and in the cell walls and intercellular spaces of lateral roots (63). Such apoplastic phosphatase is safe from soil but can be effective only when presented with soluble organophosphates, which are often present in the soil. solution (64). However, because the phosphatase activity in the rhizo-sphere originates from a number of sources (65), mostly microbial, and is much higher in the rhizosphere than in bulk soil (66), it seems curious that plants would have a need to secrete phosphatase at all. 

D. L. Jones and P. R. Darrah, Re-sorption of organic compounds by roots of Zea mays L. and its consequences in the rhizosphere I. Re-sorption of C labelled glucose, mannose and citric acid, Plant and Soil 143 259 (1992). 

Factors that affect the accessibility of chemicals to plant roots include hydrophobicity, polarity, sorption properties and solubility. In order to apply phytoremediation techniques to soils polluted by organic contaminants, the contaminant must come into contact with the plant roots and be dissolved... 

This solution is valid for the initially linear portion of the sorption (or desorption) curve when MtIM is plotted against the square root of time. These equations also demonstrate that for Fickian processes the sorption time scales with the square of the dimension. Thus, to confirm Fickian diffusion rigorously, a plot of MJM vs. Vt/T should be made for samples of different thicknesses a single master curve should be obtained. If the data for samples of different thicknesses do not overlap despite transport exponents of 0.5, the transport is designated pseudo-Fickian. ... 

The result shown in Eq. (54) is a square root of time relationship for moisture uptake. Mulski [20] demonstrated that for sodium glycinate in a hydrophobic porous matrix, moisture sorption follows Eq. (54). 

The enhancing effect of NaCl on Cd uptake was due to chloride complexation of Cd (Smolders et al., 1998). High salinity in soils increased the concentrations of chloride complexes of trace elements (such as CdCl or CdCl2°) in soil solution, which increased and correlated best with Cd uptake of both plant species as discussed above. In addition, salinity also affected plant root function, and Na competition with Cd for sorption sites in soil may be a possible contributor. 

Sacan MT, Balcioglu IA. 1996. Prediction of the soil sorption coefficient of organic pollutants by the characteristic root index model. Chemosphere 32 1993-2001. 

Metal cations in the soil solution may be immobilized by sorption onto iron plaque on root surfaces in submerged soils, in the same way that solubilized Zn + was re-adsorbed on ferric oxide in the experiments in Figure 6.22. Sequestering of metals on the external surfaces of wetland roots in this way limits uptake... 

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